1. Field of the Invention
The present invention relates to an optical waveguide forming epoxy resin composition to be used as a material for a cladding layer of an optical waveguide of an optical waveguide apparatus widely used for optical communications, optical information processing and other general optics.
2. Description of the Related Art
Optical waveguide cladding materials for light transmission flexible printed boards are required to have high flexibility, low refractive index and excellent patternability. In designing a material satisfying such requirements, an aliphatic resin is typically selected for the lower refractive index, and a multi-functional aliphatic epoxy resin and a long-chain bi-functional aliphatic epoxy resin are blended as required to impart the material with excellent patternability (high sensitivity) and high flexibility. For a cladding material particularly required to have higher flexibility, therefore, the amount of the long-chain bi-functional aliphatic epoxy resin is inevitably increased, so that the cladding material tends to have a lower glass transition temperature Tg after being cured (see, for example, JP-A-2011-27903 and JP-A-2010-230944).
In a roll-to-roll process for mass production, on the other hand, a dry film technique is generally employed, in which an uncured film is used in the form of a dry film material. In order to adapt the dry film material for the roll-to-roll process, the uncured material is required to have lower tackiness and flexibility. In development of the material, however, these requirements reduce the material design flexibility. In production of the dry film, it is necessary to provide laminate substrates on opposite surfaces of the dry film, resulting in problems associated with resource saving and cost saving. In the material development, it is also important to adapt the material for a wet process in which a liquid material is applied onto a substrate for formation of a clad and a core (JP-A-2010-230944).
In the wet process, a surface smoothing agent (leveling agent) is added to the material or a solvent having a higher boiling point is used to increase the surface smoothness of the coating film in a coating film forming and solvent drying step.
However, the addition of the leveling agent reduces the surface tension of the surface of the coating film, resulting in repellency in the subsequent coating film forming step. This often adversely influences a waveguide loss.
Where the higher boiling point solvent is used instead of the leveling agent, on the other hand, the following problem will be encountered. Where a cladding material imparted with the flexibility and hence having a reduced glass transition temperature Tg is used, a core resin component is liable to infiltrate into the resulting under-cladding film due to the drying of the higher boiling point solvent in a core layer forming and drying step in which a core layer is formed on the under-cladding film. Therefore, light propagating through the core layer is liable to leak into the under-cladding film, thereby increasing the waveguide loss.
With such technical background, there is an eager demand for a cladding material having a higher glass transition temperature Tg, higher flexibility and excellent patternability for formation of a cladding layer.